It is well-known to deliver so-called oilfield chemicals (using this common term to include chemicals used in connection with either natural gas or oil and to include biochemicals such as nucleic acids and enzymes) to a subterranean hydrocarbon reservoir to bring about a variety of functions at various stages of hydrocarbon production. Methods for delivering oilfield chemicals to a reservoir include methods in which the chemical is made into the form of particles which are suspended in the fluid which is pumped down a wellbore to the reservoir. Common methods for forming particles are absorption into the pores of porous carrier particles and encapsulation as a core-shell structure in which a single quantity (the core) of the oilfield chemical is enclosed within a shell of carrier material.
Hydraulic fracturing is a well established technique for stimulating production from a hydrocarbon reservoir. In a conventional fracturing procedure a thickened aqueous fracturing fluid is pumped into the reservoir formation through a wellbore and opens a fracture in the formation. Thickened fluid is then also used to carry a particulate proppant into the fracture. Once the fracture has been made and packed with proppant, pumping is stopped. The formation closes onto the proppant pack and oil or gas can flow through the proppant pack to the wellbore. At least some of the aqueous fracturing fluid in the wellbore will be driven back to the surface by fluid produced from the reservoir. Thickener which increases the viscosity of the fracturing fluid may be a polysaccharide. Guar gum, often crosslinked with borate or a zirconium compound, is frequently used. Another category of thickeners which is used is viscoelastic surfactants. An oilfield chemical may be delivered to a reservoir during fracturing. If the fracturing fluid contains a viscosifying thickener, it is normal to supply a so-called breaker (which is usually a chemical or an enzyme) into the fracture to degrade the thickener and to reduce the viscosity of the fluid in the fracture after it has served its purpose. This facilitates the flow back to the surface and the flow of produced fluid through the proppant pack towards the wellbore.
U.S. Pat. No. 4,506,734 teaches the encapsulation of a breaker chemical, which may be an enzyme, within particles which are crushed by the fracture when pumping ceases and the fracture is allowed to close onto the proppant pack. This of course delays release of the breaker chemical until the fracture formation is complete.
U.S. Pat. No. 5,437,331 discloses an encapsulated enzyme breaker formed by a procedure in which porous beads of polymer are made and exposed to a solution of the enzyme which is absorbed into the open pores of the polymer beads. Examples in the document showed that this delayed release of the enzyme compared to incorporating enzyme solution directly into a model fracturing fluid.
Proposals for encapsulation of oilfield chemicals in contexts other than hydraulic fracturing include U.S. Pat. No. 6,818,594 which teaches the use of enzymes which are enclosed within a polymer capsule as a breaker for filtercake formed while drilling a well.
WO 03/106809 teaches particles in which an oilfield chemical (in the form of small droplets of aqueous solution) is enclosed in a matrix of an encapsulating polymer. This polymer is chosen so as to be soluble or otherwise degradable under conditions which are encountered within the reservoir after mixing with formation fluid found in the reservoir. The document teaches that these encapsulated particles should be made so small (mean particle diameter below 10 micron) that they can enter the pores of formation rock. Although delivery of particles to a reservoir via a production well is mentioned, an alternative possibility which is suggested is that particles can be delivered to the reservoir via an injection well and then flow through the formation to the vicinity of a production well to release the encapsulated chemical (a scale inhibitor) in the near wellbore region of the production well. This indicates that release of the encapsulated chemical will be sufficiently slow to allow time for travel through the formation from the injection well to the production well.